It is at least partially due to it being the end of World War II, since one way Germany was punished was through the removal of its territory. Probably because muh Nazis, uprooting millions of ordinary people from their ancestral homelands and forcing them out was seen as less immoral. Germany of course deserved punishment for World War II, though I think too much territory was taken (i.e. giving Poland land it hadn't had for almost 1,000 years) and there shouldn't have been so many people forced out.
Is it ever right to evict an ethnic group?
The German population, in what had formerly been Prussia, was nearly destroyed. An entire culture was wiped out. The remnants of Prussian culture was dissolved in East Germany, which obviously had a culture that was the complete opposite of Prussian. The scapegoating of Germany in WWI may have been the main cause of tens of millions of deaths and more forced migrations.
I think the reasons why it's ignored are rather obvious and understandable. That doesn't make it any less bad, of course.
Though I normally consider myself understanding, the largest forced migration in all of European history is, strangely, hard for me to understand.